Delta-Sns: An Emerging Bidirectional Auxetic Direct Semiconductor With Desirable Carrier Mobility And High-Performance Catalytic Behavior Toward The Water-Splitting Reaction

We propose a novel two-dimensional SnS allotrope (monolayer delta-SnS) based on an auxetic delta-phosphorene configuration using first-principles calculations. This monolayer appears to have outstanding stability as revealed by its energetic, kinetic, thermodynamic, and mechanic calculations, and it can withstand temperatures as high as 900 K. Monolayer delta-SnS is a wide direct-bandgap (2.354 eV) semiconductor, and its electron mobility is as high as similar to 1.25 x 10(3) cm(2) V-1 s(-1), higher than that of monolayer KTlO (similar to 450 cm(2) V-1 s(-1)) and MoS2 (similar to 200 cm(2) V-1 s(-1)). Optical absorption spectra, reaching up to the order of similar to 10(5) cm(-1), are obviously excellent in the visible-light region, suggesting efficient harvesting of solar radiation. Because of its unique atomic motif, monolayer delta-SnS presents an unusual bidirectional auxetic effect: a high negative in-plane Poisson's ratio (-0.048 and -0.068), which is larger than those for many recently reported two-dimensional auxetic materials, e.g., black phosphorene (-0.027), borophene (-0.04), and monolayer penta-B2N4 (-0.02). The bandgap and band edge can be substantially manipulated under strain to meet the requirement of the water-splitting reaction. Particularly, when pH = 7, suitable band-edge alignments and small overpotentials of the photocatalytic OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) appear, endowing monolayer delta-SnS with great potential as an efficient visible-light-driven bifunctional photocatalyst for water splitting.